CHAPTER 12 SEX-LINKED TRAITS

Karyotype: a picture of chromosomes.

Body Cells vs. Sex Cells

Autosomes: the first 22 homologous pairs of chromosomes.

Autosomes are the same for both males and females.

Sex cells: Determine sex of offspring

Is it Male or Female? Explain

Sex chromosomes: determines the sex of the individual.

The sex chromosomes are the 23rd pair of chromosomes.  XX =female

XY=male

Which parent determines the sex of an offspring?

DADWhy? All moms have the genotype XX.

When egg cells are made, they will all carry a single X chromosome.

All dads have the genotype XY. When sperm cells are made, 50% will have an X chromosome and 50% will have a Y chromosome.

Therefore, males and females are born in roughly a 50:50 ratio.

SEX-LINKED TRAITS

Those traits that are controlled by genes on the X or Y chromosomes.

NOTE: The Y chromosome is much smaller than the X chromosome and only contains a few genes. Most sex-linked traits are on the X chromosome.

In humans, hemophilia is a sex-linked trait.  Having hemophilia is recessive (Xh) to being normal (XH). The heterozygous female is called a carrier.  Cross a carrier female with a normal male.

mal

e__ XHXh __  X  __ XHY ___

XH XH

XH Y Xh Y

XH Xh

XH

Y

XH Xh

Results:Genotypic ratio: 1 XHXH :1 XHXh : 1 XHY :1 Xh Y

XH XH

Normal Female

XH Y

Normal Male

Xh YMale w/

hemophilia

XH Xh

Normal FemaleXH

Y

XH XhPhenotypic ratio:2 normal females: 1normal male: 1 male with hemophilia

Now You Try

Cross a carrier female with a male with hemophilia.

__ XHXh __  X  __ XhY ___

Results:Genotypic ratio: 1 XHXh :1 XhXh : 1 XHY :1 Xh Y

XH Xh

Normal Female

XH Y

Normal Male

Xh YMale w/

hemophilia

Xh Xh

Female w/ hemophilia

Xh

Y

XH XhPhenotypic ratio:1 normal female: 1 female w/ Hemopholia:1normal male: 1 male with hemophilia

Genetic Sex-Linked Disorders Color Blindness

Cause: x-linked recessive

1/10 males have, 1/100 females have. Why the difference?

Individuals are unable to distinguish shades of red-green.

                                          

Genetic Sex-Linked Disorders

2- Hemophilia A and B

Recessive disorders that affect 1 of 5,000 males. These interfere with normal blood clotting and occur on the X chromosome.

Using Fruit Flies to Study Genetics

Thomas Hunt Morgan, 1908 Among normal red-eyed Drosophila he found a

mutated white-eyed fly (male). Since it was a recessive gene that was expressed

only in male flies, Morgan suggested that the chromosome was located on the X chromosome.

Miniature winged mutation was transmitted on the same gene. These two characteristics did not always travel together.

Morgan suggested that the chromosomes could swap parts and received the Nobel prize in 1934.

Drosophila melanogaster

X-linked Inheritance

A situation where the genes that cause a disorder are located on the X chromosome.

More male victims than female– Hemophilia– Color-blindness– Duchenne muscular dystrophy

Polygenic Inheritance

Traits that are influenced by several Genes.

Show many degrees of variation.Examples: Height Hair color Skin Color Eye Color